Defining rate dependence for a crushable foam plasticity model

As strain rates increase, many materials show an increase in the yield stress. For many crushable foam materials this increase in yield stress becomes important when the strain rates are in the range of 0.1–1 per second and can be very important if the strain rates are in the range of 10–100 per second, as commonly occurs in high-energy dynamic events.

Context:

For more information on strain rate dependence, see Crushable Foam Plasticity Models, and Rate-Dependent Yield.

  1. Create a material model as described in Specifying a crushable foam model.”
  2. From the Suboptions menu in the Edit Material dialog box, select Rate Dependent.

    A Suboption Editor appears.

  3. Click the arrow to the right of the Hardening field, and select a method for defining hardening dependencies:

    • Select Power Law to define yield stress ratios with the Cowper-Symonds overstress law.

    • Select Tabular to enter yield stress ratios directly in tabular form as a function of equivalent plastic strain rates.

  4. If applicable, toggle on Use temperature-dependent data to define data that depend on temperature.

    A column labeled Temp appears in the Data table.

  5. If applicable, click the arrows to the right of the Number of field variables field to increase or decrease the number of field variables on which the data depend.
  6. If you selected Power Law from the list of Hardening options, enter the following data in the Data table:

    Mulitiplier

    Material parameter, D.

    Exponent

    Material parameter, n.

    Temp

    Temperature.

    Field n

    Predefined field variables.

    You may need to expand the dialog box to see all the columns in the Data table. For detailed information on how to enter data, see Entering tabular data.

  7. If you selected Yield Ratio from the list of Hardening options, enter the following data in the Data table:

    Yld Stress Ratio

    Yield stress ratio, R=σ¯/σ0.

    Eq Plastic Strain Rate

    Equivalent plastic strain rate, ε¯˙p⁢l (or |ε˙a⁢x⁢i⁢a⁢lp⁢l|, the absolute value of the axial plastic strain rate in uniaxial compression, for the crushable foam model).

    Temp

    Temperature.

    Field n

    Predefined field variables.

    You may need to expand the dialog box to see all the columns in the Data table. For detailed information on how to enter data, see Entering tabular data.

  8. Click OK to return to the Edit Material dialog box.